Seed-blanket reactors

ABSTRACT

Seed-blanket type nuclear reactor cores (10,100) are employed to burn thorium fuel with conventional reactor fuels, including nonproliferative enriched uranium, and weapons or reactor grade plutonium. In a first embodiment, the core (10) is completely nonproliferative in that neither the reactor fuel, nor the generated waste material, can be used to manufacture nuclear weapons. In a second embodiment of the invention, the core (100) is employed to burn large amounts of weapons grade plutonium with the thorium, and provides a convenient mechanism by which stockpiled weapons grade plutonium can be destroyed and converted into electrical energy. The cores of both embodiments are comprised of a plurality of seed-blanket units (12, 102) which have centrally located seed regions (18,104) that are surrounded by annular blanket regions (20,106). The seed regions contain the uranium or plutonium fuel rods (22,110), while the blanket regions contain thorium fuel rods (26,118). The moderator/fuel volume ratios and relative sizes of the seed and blanket regions are optimized so that neither embodiment generates waste materials that can be employed for manufacturing nuclear weapons. A novel refueling scheme is also employed with the first embodiment to maximize seed fuel utilization, and further insure that the spent fuel cannot be employed for manufacturing nuclear weapons.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is division of application No. 08/516,130, filed Aug.17, 1995, now U.S. Pat. No. 5,737,375, which is a continuation-in-partof application No. 08/288,749, filed Aug. 16, 1994, now abandoned.

TECHNICAL FIELD

The present invention relates in general to light water nuclear reactordesigns which employ thorium as a fuel. The reactors can burn with thethorium, nonproliferative enriched uranium, weapons grade plutonium orreactor grade plutonium.

BACKGROUND ART

Nuclear power remains an important energy resource throughout the worldtoday. Many countries without sufficient indigenous fossil fuelresources rely heavily on nuclear power for the production ofelectricity. For many other countries, nuclear energy is used as acompetitive electricity producer that also diversifies their energy mix.Further, nuclear power also makes a very important contribution to thegoals of controlling fossil fuel pollution (e.g., acid rain, globalwarming), and conservation of fossil fuels for future generations. Interms of numbers, nuclear power provides approximately 11% of theworld's electricity. At the end of 1994, there were 424 nuclear powerplants in 37 countries. Plants under construction will bring this numberto approximately 500 plants by the end of the decade.

Although safety is certainly a major concern in the design and operationof nuclear reactors, another major concern is the threat ofproliferation of materials which could be used in nuclear weapons. Thisis of particular concern in countries with unstable governments whosepossession of nuclear weapons could pose a significant threat to worldsecurity. Nuclear power must therefore be designed and used in a mannerwhich does not cause proliferation of nuclear weapons, and the resultingrisk of their use.

Unfortunately, all present nuclear power reactors create large amountsof what is known as reactor grade plutonium. For example, a typical1,000 MWe reactor creates on the order of 200-300 kg per year of reactorgrade plutonium. It is not difficult to reprocess this dischargedreactor grade plutonium into weapons grade plutonium, and onlyapproximately 7.5 kg of reactor grade plutonium is required tomanufacture a single nuclear weapon. Accordingly, the fuel dischargedfrom the cores of conventional reactors is highly proliferative, andsafeguards are required to insure that the discharged fuel is notacquired by unauthorized individuals. A similar security problem existswith the vast stockpiles of weapons grade plutonium which have beencreated as the U.S. and the countries of the former U.S.S.R. havedismantled their nuclear weapons.

Other problems involved with the operation of conventional nuclearreactors concern permanent disposal of long term radioactive wasteproducts, as well as the quickly diminishing worldwide supply of naturaluranium ore. Regarding the former, government owned repository spacesare virtually nonexistent and the Yucca Flats project located in theUnited States has now been delayed by Congress. As to the latter,significant problems with supplies of natural uranium ore are foreseenwithin the next 50 years.

As a result of the foregoing problems, attempts have been made in thepast to construct nuclear reactors which operate on relatively smallamounts of nonproliferative enriched uranium (enriched uranium having aU-235 content of 20% or less), and do not generate substantial amountsof proliferative materials, such as plutonium. Examples of such reactorsare disclosed in my two previous international applications, Nos.PCT/US84/01670, published on 25 Apr. 1985 under InternationalPublication No. WO 85/01826, and PCT/US93/01037, published on 19 Aug.1993 under International Publication No. WO 93/06477. The '826 and '477applications both disclose seed-blanket reactors which derive asubstantial percentage of their power from thorium fueled blankets. Theblankets surround an annular seed section which contains fuel rods ofnonproliferative enriched uranium. The uranium in the seed fuel rodsreleases neutrons which are captured by the thorium in the blankets,thereby creating fissionable U-233 which burns in place, and generatesheat for powering the reactor.

The use of thorium as a nuclear reactor fuel in the foregoing manner isattractive because thorium is considerably more abundant in the worldthan is uranium. In addition, both of the reactors disclosed in the '826and '477 applications claimed to be nonproliferative in the sense thatneither the initial fuel loading, nor the fuel discharged at the end ofeach fuel cycle, is suitable for use in the manufacture of nuclearweapons. This is accomplished by employing only nonproliferativeenriched uranium as the seed fuel, selecting moderator/fuel volumeratios which minimize plutonium production and adding a small amount ofnonproliferative enriched uranium to the blanket whose U-238 componentuniformly mixes with the residual U-233 at the end of the blanket cycle,and "denatures" the U-233, thereby rendering it useless for manufactureof nuclear weapons.

Unfortunately, Applicant has discovered through continued research thatneither of the reactor designs disclosed in the aforementionedinternational applications is truly nonproliferative. In particular, ithas now been discovered that both of these designs result in a higherthan minimum production of proliferative plutonium in the seed due tothe annular seed arrangement. The use of the annular seed with both aninner, central blanket section and an outer, surrounding blanket sectioncannot be made nonproliferative because the thin, annular seed has acorrespondingly small "optical thickness" which causes the seed spectrumto be dominated by the much harder spectrum of the inner and outerblanket sections. This results in a greater fraction of epithermalneutrons and a higher than minimum production of proliferative plutoniumin the seed.

Both of these previous reactor designs are also not optimized from anoperational parameter standpoint. For example, moderator/fuel volumeratios in the seed and blanket regions are particularly crucial tominimize plutonium production in the seed, permit adequate heat removalfrom the seed fuel rods and insure optimum conversion of thorium toU-233 in the blanket. Further research indicates that the preferredmoderator/fuel ratios disclosed in these international applications weretoo high in the seed regions and too low in the blanket regions.

The previous reactor core designs were also not particularly efficientat consuming the nonproliferative enriched uranium in the seed fuelelements. As a result, the fuel rods discharged at the end of each seedfuel cycle contained so much residual uranium that they needed to bereprocessed for reuse in another reactor core.

The reactor disclosed in the '477 application also requires a complexmechanical reactor control arrangement which makes it unsuitable forretrofitting into a conventional reactor core. Similarly, the reactordisclosed in the '826 application cannot be easily retrofitted into aconventional core either because its design parameters are notcompatible with the parameters of a conventional core.

Finally, both of the previous reactor designs were designed specificallyto burn nonproliferative enriched uranium with the thorium, and are notsuitable for consuming large amounts of plutonium. Thus, neither ofthese designs provides a solution to the stockpiled plutonium problem.

DISCLOSURE OF INVENTION

In view of the foregoing, it is an object of the present invention toprovide improved seed-blanket reactors which provide optimum operationfrom both an economic and a nonproliferative standpoint.

It is a further object of the present invention to provide seed-blanketreactors which can be easily retrofitted into conventional reactorcores.

It is another object of the present invention to provide a seed-blanketreactor which can be utilized to consume large quantities of plutoniumwith thorium, without generating proliferative waste products.

A still further object of the present invention is to provideseed-blanket reactors which produce substantially reduced amounts ofhigh level radioactive wastes, thereby resulting in a significantreduction in long term waste storage space requirements.

The foregoing and other objects of the invention are achieved throughprovision of improved seed-blanket reactors which utilize thorium fuelin combination with either uranium or plutonium fuel. The firstpreferred embodiment of the present invention comprises an improvedversion of the nonproliferative reactor disclosed in the '477application. Through the use of specific moderator to fuel ratios and anovel refueling scheme, this embodiment of the invention achieves a fuelburn up efficiency which has heretofore been impossible to achieve inany known reactors, and generates only nuclear wastes that are incapableof being used for formation of nuclear weapons. A second preferredembodiment of the invention is designed specifically for consuming largequantities of both reactor grade discharge plutonium and weapons gradeplutonium in a fast, efficient manner. Again, the waste materialgenerated thereby cannot be employed for forming nuclear weapons.

The first embodiment of the invention is known as the nonproliferativelight water thorium reactor, and is so named because neither its fuelnor its waste products can be employed for forming nuclear weapons. Thenonproliferative reactor's core is comprised of a plurality ofseed-blanket units (SBUs), each of which includes a centrally locatedseed region and a surrounding, annular blanket region. The SBUs arespecifically designed to be easily retrofitted in place of fuelassemblies of a conventional reactor core.

The seed regions in the SBUs have a multiplication factor greater than1, and contain seed fuel elements of enriched uranium with a ratio ofU-235 to U-238 equal to or less than 20% U-235 to 80% U-238, this beingthe maximum ratio which is considered to be nonproliferative. Theenriched uranium is preferably in the form of rods and/or platesconsisting of uranium-zirconium alloy (uranium-zircalloy) or cermet fuel(uranium oxide particles embedded in a zirconium alloy matrix).

The blanket regions have a multiplication factor less than 1, andcontain blanket fuel elements essentially comprising Th-232 with a smallpercentage of enriched uranium (again enriched as high as 20% U-235) toassist the seed in providing reactor power during the initial stages ofoperation when the thorium is incapable of providing power on its own.By adding enriched uranium to the blanket, the blanket can generateapproximately the same fraction of power at start up that it does laterwhen a large number of neutrons released by the seed fuel elements havebeen absorbed by the thorium fuel elements in the blanket. Thisabsorption generates fissionable U-233 which is burned in place, andprovides power from the blanket once the reactor is up and running.

The 20% enriched uranium oxide in the blanket also serves to denatureany residual U-233 left in the blanket at the end of its lifetime byuniformly mixing the U-233 with nonfissionable uranium isotopesincluding U-232, U-234, U-236 and U-238. This denaturing is importantbecause it is nearly impossible to separate the residual U-233 from thenonfissile isotopes thus making the residual U-233 unsuitable for use inthe formation of nuclear weapons.

Light water moderator is employed in both the seed and blanket regionsof each SBU to control reactivity. Unlike in conventional uranium cores,boron is not dissolved in the water moderator during power operationbecause this would unacceptably lower the multiplication factor of theblanket, thus resulting in a drastically lower blanket power fraction.

The volume ratios of the water moderator to fuel in each region arecrucial. In the seed region, to insure that the reactor will notgenerate sufficient amounts of plutonium waste to be consideredproliferative, the moderator/fuel ratio must be as high as practicableto slow down the neutrons in the seed, and decrease the likelihood thatthey will be absorbed by the uranium-238 in the seed, thereby generatingplutonium. Unfortunately, to increase the moderator volume in the seednaturally implies that the fuel volume must be correspondinglydecreased, and this increases the power density which, if increased toofar, will generate too much heat. Both of these factors must thereforebe taken into consideration in order to determine the optimummoderator/fuel ratio in the seed region. Use of uranium/zirconium alloyfor the seed fuel permits a higher moderator/fuel ratio because of itshigher thermal conductivity compared to that of oxide fuel. Using thesetypes of fuel elements, the moderator/fuel ratio in the seed regionshould be between 2.5 and 5.0, and preferably between 3.0 and 3.5.Another benefit of the use of the high moderator/fuel ratio in the seedis that it results in a substantial reduction in the generation of highlevel radioactive wastes, particularly transuranic actinides. This,combined with the fact that the blanket fuel rods remain in the core forapproximately 10 years, results in a substantial reduction in long termwaste storage space requirements.

The moderator/fuel volume ratio in the blanket region should beconsiderably lower than that in the seed region because it is desirablethat the thorium fuel in the blanket absorb as many neutrons aspossible. These are necessary to convert the thorium into fissionableU-233 which is burned in place, and supplies a substantial portion ofthe reactor power. Research has established that the optimummoderator/fuel volume ratio in the blanket region should be in the rangeof approximately 1.5-2.0, and preferably approximately 1.7. If the ratiois higher than 2.0, too many thermal neutrons will be absorbed by thewater, while if the ratio is below 1.5, too much protactinium will beformed in the blanket region which will also interfere with theformation of U-233.

A once-through fuel cycle is employed with the first preferredembodiment which eliminates the need for reprocessing spent fuelassemblies for future use. In addition, a novel refueling scheme isemployed which maximizes fuel consumption in both the seed and blanketregions, and further reduces the likelihood that any of the fuelremaining in the spent fuel elements can be reprocessed and employed inthe manufacture of nuclear weapons. In this refueling scheme, the seedfuel elements are replaced in a staggered manner in which a portion,preferably 1/3, of the total seed fuel elements is replaced at the endof each fuel cycle, and each seed fuel element remains in the core formore than one, preferably three, fuel cycles. Each fuel cycle isapproximately 13 months in length. The blanket fuel elements, becausethey are comprised predominantly of thorium, can remain in the core forup to nine fuel cycles, or approximately 10 years. However, shuffling ofthe SBUs in the core is performed at the end of each fuel cycle toimprove power distribution throughout the core.

This refueling scheme enables the enriched uranium seed fuel rods to bedepleted down to less than 20% of their original U-235 content. Inaddition, the long residency time in the core of the seed fuel elementsincreases the generation of Pu-238 to the point where it denatures therelatively small amount of Pu-239 which is generated by the seed fuelelements. As a result, the spent seed fuel elements are effectivelyrendered useless for the formation of nuclear weapons.

The second preferred embodiment of the present invention uses the samebasic seed-blanket core arrangement as the first preferred embodimentwith a plurality of SBUs that can be retrofitted into a conventionalreactor core. However, this embodiment of the invention is designedspecifically for consuming very large amounts of plutonium, eitherweapons grade or reactor discharge grade, with the thorium in theblanket. Thus, the thorium oxide is mixed with plutonium in the blanketfuel rods, while the seed fuel rods are formed predominantly ofplutonium-zirconium alloy. Unlike the first embodiment whose goal is tomaximize the amount of power generated by the thorium in the blanket,the goal of the second embodiment is to maximize the consumption ofplutonium without generating large amounts of new plutonium as typicallyoccurs in a conventional reactor.

The plutonium incinerator embodiment also employs a high watermoderator/fuel volume ratio, preferably between approximately 2.5 and3.5. However, the reason for the high ratio is different than that forthe first embodiment. In particular, the high water to fuel volume ratioprovides a very thermal spectrum in the seed regions. This simplifiescore control since all control is concentrated in the seed regions, andcontrol can thereby be effected without boron chemical control orincreased use of control rods.

In the blanket region, the only notable difference in the plutoniumincinerator embodiment is that the thorium oxide in the blanket fuelrods is mixed with a small percentage of plutonium oxide to assistduring initial reactor operation. In addition, it is very important thatapproximately 2-5% by volume uranium tailings (natural uranium with itsU-235 content reduced to approximately 0.2%) are added to the blanketfuel rods. These tailings serve to denature (render useless for use inthe manufacture of nuclear weapons) the U-233 which is formed in theblanket during reactor operation. The moderator/fuel ratio in theblanket region is preferably between approximately 1.5 and 2.0 tosatisfy neutronic and thermal hydraulic constraints.

BRIEF DESCRIPTION OF THE DRAWINGS

The features and advantages of the present invention will becomeapparent from the following detailed description of a number ofpreferred embodiments thereof, taken in conjunction with theaccompanying drawings, in which:

FIG. 1 is a schematic cross sectional illustration of a nuclear reactorcore constructed in accordance with a first preferred embodiment of thepresent invention known as the nonproliferative light water thoriumreactor;

FIG. 2 is a detailed cross sectional illustration of a seed-blanket fuelassembly unit (SBUs) employed in the first preferred embodiment;

FIG. 3 is a partial cross sectional illustration of an SBU modified toinclude burnable poison rods for reactor control;

FIG. 4 is a graph illustrating the reactivity level as a function offull powered days for the first seed fuel cycle of a number ofvariations of the modified SBU illustrated in FIG. 3;

FIGS. 5.1-5.9 are fuel loading maps corresponding to each of ninedifferent seed fuel cycles that are employed during operation of thereactor core illustrated in FIG. 1;

FIG. 6 is a schematic cross sectional illustration of a reactor coreconstructed in accordance with a second preferred embodiment of theinvention known as the plutonium incinerator;

FIG. 7 is a detailed cross sectional illustration of an SBU employed inthe second preferred embodiment; and

FIG. 8 is a core map illustrating the reload configuration andaccumulated burnup for the second preferred embodiment.

BEST MODE FOR CARRYING OUT THE INVENTION A. The Nonproliferative LightWater Thorium

Nuclear Reactor

Turning now to a detailed consideration of a first preferred embodimentof the present invention known as the nonproliferative light waterthorium nuclear reactor, FIG. 1 illustrates a nuclear reactor core 10comprised of a plurality of fuel assemblies 12, known as seed-blanketunits (SBUs), that are arranged in a generally hexagonal configuration,and are themselves hexagonal in cross section. The core 10 is of thesame geometrical configuration and dimensions as a conventional Russianlight water reactor known as the VVER-1000 so that it can be easilyretrofitted into a VVER-1000, and is formed of 163 of the SBU fuelassemblies 12. The difference between the core 10 and the VVER-1000reactor core lies in the composition of the SBUs 12 as will be discussedin greater detail below. It will be understood that the shape andarrangement of the core 10 and the SBUs 12 can be modified as necessaryto facilitate retrofitting into any type of conventional light waterpressurized water reactor (PWR). For example, conventional PWRs in theUnited States and other countries employ fuel assemblies having squarecross sections, and the SBUs 12 would also have square cross sections ifthey were designed to be retrofitted into such a PWR.

Surrounding the core 10 is a reflector 14 which is preferably comprisedof a plurality of reflector assemblies 16 as illustrated in FIGS. 1 and5.1-5.9. Each of the reflector assemblies 16 preferably contains amixture of water and core barrel/pressure vessel metal. Alternatively,each of the reflector assemblies 16 could also be formed predominantlyof thorium oxide.

FIG. 2 illustrates the composition of each of the SBU fuel assemblies12. Each of the SBUs 12 includes a centrally located seed region 18 andan annular blanket region 20 which surrounds the seed region 18. Theseed region 18 is comprised of a plurality of seed fuel rods 22 whichare preferably formed of uranium-zirconium alloy containing U-235/U-238initially enriched to as high as 20% U-235, which is the maximumenrichment that is considered to be nonproliferative, i.e., incapable ofbeing utilized to manufacture nuclear weapons. While it is not necessaryto maximize the initial U-235 enrichment to 20%, it is preferable toemploy this enrichment level to minimize plutonium production in theseed during reactor operation. Alternatively, the fuel rods 22 can bemade of cermet fuel with uranium oxide particles embedded in a zirconiumalloy matrix. The use of zirconium alloy (zircalloy) in the seed fuelrods 22 is preferred over oxide type fuel because the zirconium alloyfuel has a much higher thermal conductivity. As will be discussed ingreater detail below, this is important because it reduces the amount ofspace needed in the SBU 12 for heat removal, and thereby increases theamount of space available for water moderator. The seed region 18 alsocontains a plurality of water tubes 24 for reception of water moderator(or conventional burnable poison rods and/or control rods as discussedin greater detail below) to control reactivity in the seed region 18.

The blanket region 20 contains a plurality of blanket fuel rods 26 whichare preferably formed of mixed thorium-uranium oxide. The initialuranium oxide volume content in the thorium-uranium mixture ispreferably in the range of approximately 2-10%, and is employed to helpfuel the blanket region 20 on start up before the thorium has had achance to absorb neutrons from the seed, and generate the blanket's ownfissile fuel, U-233. As in the seed fuel rods 22, the uranium oxidecontained in the blanket fuel rods 24 is preferably U-235/U-238 enrichedinitially as high as the maximum nonproliferative ratio of 20:80.

The seed-blanket core 10 operates in accordance with the followingsimplified equation for the power sharing between the seed 18 and theblanket 20:

    P.sub.b /P.sub.s =ε(K.sub.b /(1-K.sub.b))(K.sub.s-1)/K.sub.s

In the foregoing equation, K_(s) and K_(b) are the multiplicationfactors of the seed and blanket respectively. P_(s) and P_(b) are thepowers generated in the seed and blanket respectively, while ε is thefast effect, which is slightly over 1. The seed multiplication factor,K_(s), is greater than 1, and the blanket multiplication factor, K_(b),is less than 1. Thus, the blanket is subcritical, and the seed acts as asource of neutrons for the blanket.

In order to maximize the amount of energy produced from thorium, it isnecessary to make the fraction of the core power produced in the blanket20 as high as possible. This is accomplished by making K_(s) as high aspossible, and it has been determined that K_(s) can be as high as 1.70,while K_(b) is selected between approximately 0.85 and 1.

The number of neutrons absorbed by U-238 in the seed 18 must beminimized. Most of the neutrons absorbed in U-238 are in what is calledthe resonance energy region marked by closely spaced energy intervals ofextremely high absorption. On the other hand, most of the fissions inU-235 occur at lower energies in the thermal region where the averageneutron energy is in near equilibrium with the ambient temperature ofthe light water moderator. By making the water content of the seed 18 ashigh as practicable, the number of neutrons in the resonance region isdecreased, and thus, fewer neutrons are captured by the U-238.

Reduction of U-238 captures produces two favorable effects. First, themultiplication factor of the seed, K_(s), is raised, thereby increasingthe fraction of core power produced in the blanket as discussed above,and second, the formation of plutonium is minimized since it is theU-238 neutron captures which forms the plutonium.

The amount of water that can be placed in the seed region 18 is limitedby the need to have enough room for the fuel rods 22 to permit adequateheat removal from the same. The volume and surface area of the fuel rodsmust therefore not be reduced to the point where the power density inthe core rises beyond operational limits dictated by the reactor'scooling system. By fabricating the seed fuel elements 22 out ofuranium/zirconium alloy, which has a much higher thermal conductivitythan does oxide fuel, the water moderator/fuel volume ratio in the seed18 can be made as high as 4 or 5 to 1 as compared with less than 2 to 1in a conventional uranium core. The water moderator/fuel ratio in theseed 18 should therefore be selected between approximately 2.5 and 5.0,and most preferably between 3.0 and 3.5.

Another advantage to the high moderator/fuel volume ratio in the seed 18is that it substantially reduces the quantity of high level radioactivewaste generated in the seed 18. In particular, because the seed spectrumis very thermal due to the large water fraction, very few transuranic orminor actinides will be produced. It is these actinides, with half livesof millions of years, that require very long term storage in undergroundrepositories. The 10 year blanket life coupled with the reduced actinideproduction from the nonproliferative core 10 therefore produces lessradioactive waste materials and also less long term heat generation.This results in underground repository space requirements beingsignificantly reduced. In addition, low level waste is also somewhatreduced because no boric acid is dissolved in the water moderator fornormal operation, and thus no tritium is generated in the core. Itshould be noted that the reason boric acid is not employed in the watermoderator is that it would unacceptably lower the multiplication factorin the blanket region 20.

The moderator/fuel ratio in the blanket region 20 is also a veryimportant parameter, however, it is governed by different constraints.In particular, the situation in the blanket 20 is more complex becausetoo much water reduces K_(b) by absorbing too many neutrons coming fromthe seed fuel elements, and thereby taking them away from the thorium.On the other hand, too little water in the blanket increases the loss toprotactinium. When thorium absorbs a neutron, it forms protactinium,which after a 27.4 day half-life, decays into fissionable U-233. Duringthis interval, protactinium is vulnerable to absorbing a neutron andthereby forming nonfissionable U-234. This is a double loss of both aneutron and a prospective U-233 nucleus. Research indicates that tominimize this loss, the optimum value of the water/fuel ratio in theblanket 20 should be selected between approximately 1.5 and 2.0, andpreferably approximately 1.7.

Preferably, the seed region 18 comprises between approximately 25 and 40percent of the total volume in the SBU 12. This range of values is alsodetermined based upon competing considerations. First, the core 10 isdesigned to burn as much thorium as possible, thus the blanket region 20must be made as large as practical. On the other hand, the seed region18 cannot be made so small that the power density therein rises too highfor the reasons given previously. The range of 25-40 percent has beendetermined to provide the optimum balance of these competing inconsiderations.

Still another important design aspect of the SBU 12 is the centralseed/annular blanket configuration. In Applicant's previously publishedInternational Application, Publication No. WO85/01826, a seed-blanketcore is disclosed which employs an annular seed with both an inner,central blanket section and an outer, surrounding blanket section. Suchan arrangement cannot be made nonproliferative because the thin, annularseed has a correspondingly small "optical thickness" which causes theseed spectrum to be dominated by the much harder spectrum of the innerand outer blanket sections. This results in higher thermal neutronenergies, and a resulting increased production of Pu-239 in the seed.The central seed arrangement of the SBU 12 overcomes this drawback bymaking the seed section 18 thick enough to avoid excessive interactionwith thermal neutrons crossing from the blanket section 20 into the seedsection 18.

The referenced core and fuel assembly parameters for the core 10 andeach of the SBUs 12 are presented in Tables 1 and 2, respectively,below. These parameters were selected to provide a completecompatibility of the SBU fuel assembly with an existing (typical)VVER-1000 plant.

                  TABLE 1                                                         ______________________________________                                        Core Parameters                                                               Parameter                                                                     ______________________________________                                        Total Power (MWth) 3000                                                       Average Power Density                                                                            107                                                        (w/cm.sup.3)                                                                  Average Moderator Temp., °C.                                                              306                                                        Number of SBUs in Core                                                                           163                                                        Number of Control Rod                                                                             61                                                        Clusters (CRC)                                                                Number of Control Rods                                                                            12                                                        per CRC                                                                       Blanket Fuel       U + Th(O.sub.2)                                            Seed Fuel          U/Zr Alloy                                                 Seed Reload Schedule                                                                             54 Seed/Cycle                                                                 (≈1 Year)                                          Blanket Reload Schedule                                                                          163 Blankets/9                                                                Cycles (≈10 Years)                                 ______________________________________                                    

                  TABLE 2                                                         ______________________________________                                        SBU Parameters                                                                Parameter          Seed     Blanket                                           ______________________________________                                        Outer Radius of Fuel                                                                             0.310    0.380                                             Pellet, cm                                                                    Outer Radius of Gas Gap,                                                                         --       0.3865                                            cm                                                                            Outer Radius of Cladding,                                                                        0.370    0.4585                                            cm                                                                            Cell Radius, cm    0.6652   0.6731                                            Pitch, cm          1.267    1.282                                             Moderator/Fuel Volume Ratio                                                                      3.18     1.68                                              Number of Fuel Rods                                                                              156      162                                               Number of Water Tubes                                                                            12       0                                                 Number of Other Tubes                                                                            1        0                                                 Seed Total Weight, tH.M.                                                                         6.71     --                                                Blanket Total Weight, tH.M.                                                                      --       35.82                                             U (In Blanket) t   --       3.11                                              ______________________________________                                    

To provide additional reactivity control during each seed cycle, the SBU12 can be modified as illustrated in FIG. 3 to include a plurality ofburnable poison containing rods 28 and 30 which are positioned at spacedlocations in the seed section 18. In the example illustrated in FIG. 3,the first group of burnable poison rods 28 comprise standardWestinghouse burnable poison rods known as WABAs as are presentlyutilized in conventional PWR fuel systems. These rods are formed of acomposite material consisting of boron-10, boron-11, carbon, aluminumand oxygen. The second group of burnable poison containing rods 30comprise uranium/zircalloy seed fuel rods which have been modified tocontain a small percentage of natural gadolinium. Any number andcombination of the burnable poison containing rods 28 and 30 can beemployed as necessary. In the example illustrated in FIG. 3, each SBU 12contains 12 of the WABAs 28 and 6 of the gadolinium/fuel rods 30.

Both types of burnable poison rods have their advantages. The WABAsprovide a more uniform control of reactivity until the end of eachreactor fuel cycle, while the gadolinium/fuel rods 30 provide a largenegative reactivity input for the first third of the reactor cycle life.FIG. 4 illustrates the reactivity level K in each of the SBUs 12 as afunction of full power days for each of four seed control variations: nopoison, gadolinium poison, boron poison and combined gadolinium andboron poison. As illustrated, the combination of both types of poisoncontrol results in the flattest reactivity curve.

Conventional control rods are also preferably employed to compensate theexcess reactivity in the reactor core. In addition, the control rods canbe employed for emergency shutdown (scram) of the reactor andcompensation for power transients resulting from Xe oscillations andmoderator temperature transients. The control rods are assembled intocontrol rods clusters (CRCs) with 12 control rods per CRC. As noted inTable 1, it is not necessary that each of the SBUs 12 include a CRC, andcalculations indicate that it is sufficient to place place one CRC ineach of 61 of the 163 SBUs in the core.

In the operation of the nonproliferative light water thorium nuclearreactor core 10, a once-through fuel cycle is employed in which all ofthe fuel rods in both the seed and blanket regions 18 and 20 are used inthe reactor core only once. However, a unique fuel management scheme isemployed in which the seed and blanket fuel assemblies follow separatefuel management paths. In particular, each of the seed fuel rods 22remains in the reactor core for more than one seed fuel cycle(approximately 13 months), preferably three cycles, however, only afraction (preferably 1/3) of the seeds is replaced at the end of eachseed fuel cycle. Preferably, the positions of the SBUs 12 in the core 10are also shuffled at the end of each seed fuel cycle to improve thepower distribution throughout the core. In contrast, each of the blanketfuel rods 24 remains in each SBU 12 for the entire life of the blanket20, which is preferably 9 fuel cycles, or approximately 10 years.

This fuel management scheme combined with the seed-blanket arrangementand associated core parameters allows approximately 80-90% of theuranium in the seed fuel elements 22 to be consumed before they areremoved from the core 10. As a result, the spent seed fuel rods 22 areof no economic or nuclear value since so little of the original U-235loading remains.

In addition, this extended burn-up of the seed fuel rods causes abuildup of Pu-238 which is sufficiently high to completely denature thesmall amount (approximately 30 kg. per year) of Pu-239 that is producedin the seed 18. More specifically, approximately 8-9% of the totalplutonium produced by the reactor core 10 is Pu-238. Since Pu-238 is aheat generator which produces approximately 300 times the amount of heatgenerated by Pu-239, weapons grade plutonium, such a high percentage ofPu-238 prevents the plutonium produced by the reactor core from beingused for weapons purposes. In particular, numerous studies havedetermined that reactor grade plutonium cannot be used for weaponspurposes, even by refrigerating the weapons down to 0° F., where thecontent of Pu-238 equals or exceeds 4.9% by weight. At theseconcentrations, the heat generated by the Pu-238 causes the highexplosives to melt and the plutonium to eventually melt also, or atleast change phase from its normal Alpha Phase to Delta Phase. The phasechange decreases its density and substantially increases its criticalmass. Since the nonproliferative core 10 produces concentrations ofPu-238 well in excess of 4.9%, this effectively renders the dischargedplutonium essentially nonproliferative.

The multiple batch fuel management scheme is illustrated in greaterdetail in FIGS. 5.1 through 5.9 which show a pie slice section ofapproximately one-fifth of the SBUs 12 in the core 10. Each of the FIGS.5.1-5.9 shows the fuel loading map for each of the nine seed fuel cycleswhich correspond to one blanket fuel cycle. The fuel loading mapsreflect the basic approach adopted, i.e., a three batch fuel managementscheme. This means that at all cycles, with the exception of thetransient cycles one and two, there are three seed batches: fresh,once-burned and twice-burned. These are designated on the reload maps asF, O and T, respectively. Another major factor influencing the reloadpattern is the heavy use of burnable poisons which are capable ofsuppressing local power peaks. It should also be noted that the majorityof the fresh fuel is not loaded at the core periphery, but isdistributed predominantly within the middle part of the core atpositions 6, 8, 10 and 12, and near peripheral positions 20, 21, 23, 26and 32. Additional information shown in FIGS. 5.1-5.9 shows thedistribution of the U-Gd and WABA poison rods within the core. Theelaborate burnable poison distribution reflects the complexity of thereload patterns and the low leakage configurations used in this design.Those SBUs having CRCs are also indicated by a C.

At the beginning of core life, i.e., cycle one, all fresh seed fuelassemblies are loaded. In order to achieve a reasonable radial powerdistribution, three different uranium enrichments and weight fractionsare used. As indicated in FIG. 5.1, a first third of the SBUs 12contains seed fuel rods having 9.5% by volume uranium enriched to 12% byweight U-235, a second third of the SBUs 12 contain seed fuel rodshaving 14.5% by volume uranium enriched to 17% by weight U-235, and theremaining third of the SBUs 12 contain seed fuel rods having 17% byvolume uranium enriched to 20% by weight U-235. The target fresh fuelenrichment of 20% by weight of U-235 was used thereafter for each of thefollowing cycles 3-9. Thus, cycles one and two are transient cycles,while cycles 3-9 are quasi-equilibrium cycles. The fresh fuel enrichmentwas constant at 20% U-235 by weight, but the weight fraction of uraniumin the U/Zr alloy was varied to assure 300 full power days of operationwhich correspond to one seed fuel cycle. Since the reactor is notusually operated at full power during the entire fuel cycle, it isestimated that the actual length of the seed fuel cycle is approximately13 months.

B. The Plutonium Incinerator

The second preferred embodiment of the present invention is anotherseed-blanket reactor core design known as the plutonium incinerator. Asthe name implies, the goal of this embodiment of the invention is toconsume as much weapons or reactor grade plutonium as possible. This isin contrast to the goal of the first preferred embodiment of theinvention which is to derive as much energy as possible from the thoriumfuel in the blanket. As will be discussed in greater detail below, thecompletely different goal of the plutonium incinerator dictates thatcompletely different core parameters be employed.

The preferred form of the plutonium incinerator embodiment isillustrated in FIG. 6, and comprises a reactor core 100, again formedfrom a plurality of SBUs 102. The core 100 has a generally circularcross section, and 89 of the SBUs 102, each of which has a square crosssection. It should be noted once again that the size and shape of thereactor core is arbitrary, and can be varied as necessary to achieve adesired power output, and/or accommodate retrofitting into any type ofconventional core.

Each of the SBUs 102 includes a central seed region 104 and an annularblanket region 106. The total percentage of the SBU volume occupied bythe seed region 104 is chosen in this embodiment to be as large aspossible, preferably between approximately 45 and 55%, so that as muchplutonium can be burned in the seed as possible. A reflector 108 made ofany suitable material, such as thorium oxide, surrounds the core 102.

One preferred form of the SBU 102 is illustrated in FIG. 7. Asillustrated, the seed region 104 is comprised of a first plurality ofseed fuel rods 110 formed of plutonium (weapons or reactor grade) andzirconium alloy, or alternatively, cermet fuel. A plurality of waterholes 112 are uniformly spaced throughout the seed region 104 forreception of control rod pins. First and second pluralities of burnablepoison containing rods 114 and 116 are also uniformly positionedthroughout the seed region 104. The burnable poison containing rods 114are preferably formed of a mixture of the seed fuel and gadolinium.These can be of two types, the first type having a gadoliniumconcentration of 0.36 g/cc, and the second type having a gadoliniumconcentration of 0.72 g/cc. The burnable poison containing rods 116preferably comprise conventional WABA poison rods. Any combination ofthe two types of burnable poison containing rods 114 and 116 can beemployed as desired.

The blanket region 106 contains a plurality of blanket fuel rods 118formed predominantly of thorium oxide. Preferably, a small percentage,less than approximately 1% by volume, of plutonium oxide is mixed withthe thorium oxide in the blanket fuel rods 116 to keep the blanketmultiplication factor high during initial reactor operation. Inaddition, it is very important that approximately 2-5% by volume uraniumtailings (natural uranium with most of its U-235 isotope removed) areadded to the thorium to denature the U-233 which is formed in thethorium during reactor operation by nonfissile isotopes, U-236 andU-238. This is necessary because, unlike in the first preferredembodiment in which a small amount of enriched uranium is added to theblanket fuel rods which itself can generate these nonfissile isotopes,the plutonium added to the blanket fuel rods in the plutoniumincinerator embodiment is incapable of generating these nonfissileisotopes.

The moderator/fuel volume ratio in the seed region 104 is selected to bemuch higher than in a conventional reactor core, however, the reasonsfor doing so are different than in the nonproliferative embodiment ofthe present invention. In particular, the moderator/fuel ratio isselected to be between approximately 2.5 and 3.5, and preferably between2.5 and 3.0. This effect further increases the control poison reactivityworth therein, thereby making the reactor much easier to control. As inthe nonproliferative core embodiment, the moderator/fuel ratio in theblanket region is selected to be between approximately 1.5 and 2.0.

Example values for the main core and SBU parameters for the plutoniumincinerator embodiment of the present invention are provided in Tables 3and 4 below:

                  TABLE 3                                                         ______________________________________                                        Main Core Parameters                                                          Parameter            Value                                                    ______________________________________                                        Power Level, MWth    3250                                                     Number of SBUs in Core                                                                             89                                                       Equivalent Diameter of Core, cm                                                                    380                                                      Active Height of Core, cm                                                                          365                                                      Average Power Density, w/cc                                                                        78.5                                                     ______________________________________                                    

                  TABLE 4                                                         ______________________________________                                        Additional Core Parameters                                                    Parameter         Seed     Blanket                                            ______________________________________                                        Number of Fuel Rods/SBU                                                                         264      384                                                Number of Water Holes/SBU                                                                       25       0                                                  Distance Across Flats, cm                                                                       25.5     35.7                                               % of SBU Volume   51       49                                                 Fuel Pin Diameter, mm                                                                           8.7      8.7                                                Fuel Rod Diameter, mm                                                                           9.7      9.7                                                Pitch, mm         15.0     12.75                                              Moderator/Fuel Volume                                                                           2.54     1.49                                               Ratio                                                                         Fuel Type         Metallic Oxide                                                                         Composite                                          Fuel Material     2.4 Vol  0.55 Vol                                                             % Pu     % PuO.sub.2                                                          97.6 Vol 94.45-97.45                                                          % Zirc-  Vol % ThO.sub.2                                                      alloy    2.0-5.0 Vol %                                                                 U tailings                                         Core Heavy Metal Loading,                                                                       2300 Pu  60,700 Th                                          kg                         392 Pu                                                                        100 U tailings                                     ______________________________________                                    

In the operation of the plutonium incinerator core 100, the seed fuelrods 110 and the blanket fuel rods 118 reside in the core for two years,and are discharged simultaneously. This fuel reload scheme is optimalfrom the point of view of the plutonium inventory reduction rate, butprobably is suboptimal from the thorium utilization point of view.However, this is not a concern since the goal of the plutoniumincinerator core 100 is to maximize consumption of plutonium.

Preferably, the fuel management scheme adopts a two-batch core with astandard out-in pattern. The reload configuration and accumulated burnupfor the once and twice burnt fuel assemblies are illustrated in the coremap of FIG. 8. The accumulated burnup for the once burnt assemblies isapproximately 15 GWD/T and the discharge fuel averages approximately 31GWD/T. Three different types of fuel assemblies are illustrated in thecore map of FIG. 8. Type A assemblies employ 20 of the gadolinium basedburnable poison rods 14, each having a gadolinium concentration of 0.36g/cc, type B fuel assemblies also contain 20 of the gadolinium basedburnable poison rods 114, however, these have a gadolinium concentrationof 0.72 g/cc, and type C fuel assemblies contain 20 of the gadoliniumbased burnable poison rods 114 with a gadolinium concentration of 0.72g/cc, as well as 20 of the WABA burnable poison rods 116.

The annual charge of Pu-239 in the plutonium incinerator core 100 isapproximately 1350 kg. Each year, 500 kg of plutonium are dischargedfrom the reactor thus leaving a net destruction rate of approximately850 kg of total plutonium, although only approximately 200 kg of Pu-239remains since the rest of the remaining plutonium is in the form of theother plutonium isotopes, Pu-240, 241 and 242. An equilibrium cyclebased on a standard sized LWR fuel assembly utilizing the seed-blanketconcept will give the equivalent results.

The advantages of using the thorium fuel cycle for incinerating Pu-239in a seed-blanket reactor result from the neutronic properties ofthorium, namely its high thermal absorption cross-section. This leads toa high initial Pu inventory, and therefore to high consumption of Pu perunit energy. Driving the thorium blanket with Pu fissile material causesa high Pu power share and therefore efficient Pu incineration.

Use of a conventional homogenous light water reactor (LWR) core designpresents a controllability problem. Excess reactivity of a fuel cyclebased on Pu is of the same value of a similar uranium based cycle, whilereactivity worth of a standard control mechanism is significantly lower.The Pu-based fuel is characterized by a very high thermal absorptioncross-section, which is competing with control poison material forthermal neutrons. The results of a conventional homogeneous assemblydesign indicate that the effectiveness of control rods, soluble boronand burnable poisons is reduced by approximately a factor of 2 ascompared with conventional LWR values. The obvious solutions to thisproblem are to improve the reactivity control worth of different controlmechanisms, such as utilization of more potent absorbers and/orincreasing moderator/fuel volume ratio of the core. Unfortunately, suchsolutions have a negative impact on safety and economic performanceparameters of the reactor.

The thorium based seed-blanket design provides a unique solution to thisproblem which does not carry economic or operational penalties. Sincethe control rods and/or burnable poison rods are only positioned in theseed region 104 of each SBU 102, the control effectiveness of these issubstantially increased because the power density of the seed portion ismuch higher than that of the core average. Thus, the neutron importancefunction in the seed is very high, thereby increasing the reactivityworth of the control and poison rods. In addition, the highmoderator/fuel volume ratio in the seed region improves powerdistribution within the SBU, thereby further increasing the controlpoison reactivity worth.

C. Summary

In summary, the present invention provides two novel thorium basedseed-blanket reactor core arrangements which are particularlysignificant in that they provide economical, viable solutions to theproblems of nuclear proliferation and weapons grade nuclear fueldestruction, while at the same time providing an economic reliablesource of electrical power. The nonproliferative embodiment of thepresent invention is ideal for use by lesser developed countries becauseit eliminates any concern that the reactor fuel or waste materials willbe used for making nuclear weapons, since neither of them can be usedfor this purpose. The plutonium incinerator embodiment is particularlyattractive for use in providing an excellent means by which stockpiledweapons and reactor grade plutonium can be conveniently destroyed. Inboth embodiments, the seed-blanket core arrangement is necessary toprovide the desired results. Without it, the nonproliferative embodimentwould not work, i.e., would generate proliferative waste materials. Inthe plutonium incinerator, the seed-blanket arrangement is needed toinsure proper reactor control, and prevent generation of significant newamounts of Pu-239.

Although the invention has been disclosed in terms of a number ofpreferred embodiments, it will be understood that numerous othervariations and modifications could be made thereto without departingfrom the scope of the invention as defined in the following claims.

I claim:
 1. A nuclear reactor having a core including a plurality ofseed-blanket units, each said seed-blanket unit comprising:a) a centralseed region, said seed region containing plutonium seed fuel elements;b) a blanket region surrounding said seed region and containing blanketfuel elements comprising predominantly thorium oxide; c) moderator insaid seed region in the volume ratio of moderator to fuel in the rangeof approximately 2.5 to 3.5; and d) moderator in said blanket region inthe volume ratio of moderator to fuel of approximately 1.5 and 2.0. 2.The nuclear reactor of claim 1, wherein each of said seed fuel elementsis comprised of plutonium-zirconium alloy.
 3. The nuclear reactor ofclaim 1, wherein said seed region comprises between approximately 45 and55% of the total volume of each said seed-blanket unit.
 4. The nuclearreactor of claim 1, wherein said blanket fuel elements further includeplutonium oxide in the amount of no more than approximately 1%.
 5. Thenuclear reactor of claim 1, wherein said blanket fuel elements compriseapproximately 2-5% by volume uranium tailings.
 6. The nuclear reactor ofclaim 1, wherein the volume ratio of moderator to fuel in said seedregion is in the range of approximately 2.5 to 3.0.
 7. The nuclearreactor of claim 1, wherein said central seed region further contains aplurality of burnable poison containing rods.
 8. The nuclear reactor ofclaim 7, wherein said plurality of burnable poison containing rodsincludes WABA poison rods and gadolinium containing fuel rods.
 9. Anuclear reactor having a core including a plurality of seed-blanketunits, each said seed-blanket unit comprising:a) a central seed region,said seed region containing plutonium seed fuel elements, each of saidseed fuel elements being comprised of plutonium-zirconium alloy; b) ablanket region surrounding said seed region and containing blanket fuelelements comprising predominantly thorium oxide with approximately 1% orless plutonium oxide, and approximately 2-5% by volume uranium tailings;c) moderator in said seed region in the volume ratio of moderator tofuel in the range of approximately 2.5 to 3.5; and d) moderator in saidblanket region in the volume ratio of moderator to fuel of betweenapproximately 1.5 and 2.0.
 10. The nuclear reactor of claim 9, whereinsaid seed region comprises between approximately 45 and 55% of the totalvolume of each said seed-blanket unit.
 11. The nuclear reactor of claim10, wherein the volume ratio of moderator to fuel in said seed region isin the range of approximately 2.5 to 3.0.
 12. The nuclear reactor ofclaim 11, wherein said central seed region further contains a pluralityof burnable poison containing rods.
 13. The nuclear reactor of claim 12,wherein said plurality of burnable poison containing rods includes WABApoison rods and gadolinium containing fuel rods.